Lecture 4 - Intermittency, flexibility and energy storage

Question 1

Explain system margin

Answer 1

• To maintain reliability of supply, peak demand must not exceed production capacity at that moment.
• Historically, planners sought to ensure that installed generation capacity could meet forecast peak demand within a planning horizon.
• In liberalised markets, individual market participants are responsible only for ensuring adequate generation capacity is available to meet their own contracts to supply electricity.
• In either case, system margin can be measured as the amount by which the total installed capacity of all generating plants on the system exceeds the anticipated peak demand.

Question 2

What are the two categories of impact associated with introducing more intermittent electricity into the grid?

Answer 2

1. System balancing - relates to the relatively rapid short term adjustments needed to manage fluctuations over a time period from minutes to hours.
2. Reliability impacts - relates to the extent to which we can be confident that sufficient generation will be available to meet peak demand.

Question 3

Explain system balancing

Answer 3

Relates to the relatively rapid short term adjustments needed to manage fluctuations over a time period from minutes to hours.

Question 4

Explain reliability impacts

Answer 4

Relates to the extent to which we can be confident that sufficient generation will be available to meet peak demand.

Question 5

Explain frequency response

Answer 5

• Frequency of AC on the grid will drop in response to a sudden fall in generation or increase in demand.
• ‘Frequency response reserves’ are required to ensure frequency stays within acceptable limits (within 1% of 50Hz in the UK).

Question 6

What are frequency response reserve required for?

Answer 6

To ensure frequency stays within acceptable limits (within 1% of 50Hz in the UK).

Question 7

What is the acceptable limit for frequency response in the UK

Answer 7

Within 1% of 50Hz

Question 8

Name and describe the three categories of frequency response reserves

Answer 8

1. Enhanced frequency response (EFR) - must provide power (or reduce demand) within 1 second and provide power for a further 9 seconds.
2. Primary frequency response (PFR) - must provide power (or reduce demand) within 10 seconds and provide power for a further 20 seconds.
3. Secondary frequency response (SFR) - must provide power (or reduce demand) within 30 seconds and provide power for a further 30 mins.

Question 9

What is enhanced frequency response (EFR)?

Answer 9

Must provide power (or reduce demand) within 1 second and provide power for a further 9 seconds.

Question 10

What is primary frequency response (PFR)?

Answer 10

Must provide power (or reduce demand) within 10 seconds and provide power for a further 20 seconds.

Question 11

What is secondary frequency response (SFR)?

Answer 11

Must provide power (or reduce demand) within 30 seconds and provide power for a further 30 mins.

Question 12

What is Loss of Load Probability (LOLP)?

Answer 12

• Statistical measure of the risk of demand being unmet
• Measures the likelihood that any load (demand) is not met, and it is usually a requirement of electricity systems that LOLP is kept small.
• Variable generation increases LOLP

Question 13

What is capacity credit?

Answer 13

• The possibility for a certain power plant to increase the reliability, measured as decreased LOLP, of the power system with a certain level.
• How much electricity can a technology provide at peak demand?

Question 14

At what ratio is renewable energy currently backed up in the EU?

Answer 14

Total capacity:renewable capacity = 2:1

Question 15

What are the main costs of intermittency?

Answer 15

• Aggregate costs of intermittency are made up of additional short-run balancing costs and the additional longer term costs associated with maintaining reliability via an adequate system margin.
• A 2017 review suggested that, at variable renewable penetration of 30% or less, intermittency costs in the UK are: £5/MWh from short-run balancing costs and £4-7/MWh from costs of maintaining a higher system margin.
• Be careful summing the numbers because risks double counting some costs (e.g. system reserves to meet short-term balancing requirements may also contribute to the capacity margin).
• Balancing in the UK is mainly through thermal plants.

Question 16

Name four methods of ensuring consistent supply

Answer 16

• Energy storage
• Increased connectivity
• Demand side management
• Dispatch generation

Question 17

What impact can increased interconnectivity have on the US electricity sector?

Answer 17

By 2030, increased interconnectivity can reduce carbon emissions by up to 30% relative to 1990 without an increase in LCOE.

Question 18

Explain demand side management

Answer 18

• Modification of consumer demand through various means such as financial incentives and behavioural changes.
• Usually goal is to encourage consumer to use less energy during peak hours, or to move the time of energy use to off-peak times, e.g. night and weekend.s
• Applies to both domestic and industrial consumers.
• Can operate over a large range of timescales, each requiring different infrastructure/cooperation.

Question 19

What processes can demand side management be applied to in industry?

Answer 19

• Food: electric defrost, refrigerated warehouses, cooling production.
• Steel manufacturing/aluminium production: steel mill, electric arc furnace, oxygen generation facilities, crushing.
• Cement manufacturing: grinding mills, quarrying operations, row mix grinding, clinker grinding, crushing.

Question 20

How can energy storage be categorised?

Answer 20

• Mode of storage: electrochemical, mechanical, thermal, electrical
• Speed of response: milliseconds, seconds, minutes
• Relevant scale: off-grid (1-10s kWh), mini-grid (10-100s kWh), grid (MWh)
• Power/energy/cost/suitability
• Power/energy density

Question 21

Name the main types of electrochemical storage

Answer 21

• Lead acid battery
• Lithium-ion battery
• Redox-flow batteries
• High temperature sodium based batteries
• Power-to-gas

Question 22

Name the main types of mechanical storage

Answer 22

• Pumped hydroelectric storage (PHS)
• Compressed air energy storage (CAES)
• Flywheel

Question 23

Name the main types of thermal storage

Answer 23

• Liquid air energy storage (LAES)

Question 24

Name the main types of electromagnetic storage

Answer 24

• Supercapacitatiors

- Superconducting magnetic energy storage (SMES)

Question 25

What is the current deployment of key storage technologies?

Answer 25

1. PHS >100,000MW
2. Flywheel/Li-ion (grid) 1000MW
3. Sodium sulphur 800MW
4. CAES 700MW
5. Lead acid (grid) 300MW
6. SMES 100MW
7. Supercapacitators 50MW
8. Vanadium RFB/ PtG 20MW
9. Cryogenic thermal 8MW

Question 26

What storage can be used for the following:

• Inter-seasonal
• Seasonal
• Large-scale grid support
• Small-scale grid support
• Off-grid utility scale

Answer 26

• Inter-seasonal: Hydrogen
• Seasonal: Hydrogen/PHS/CAES
• Large-scale grid support: PHS/Battery
• Small-scale grid support: Flywheel/Battery
• Off-grid utility scale: Flywheel/Battery

Question 27

Describe flexibility in the UK electricity system

Answer 27

• Optimal 2030 UK generation mix at 50gCO2/kWh with a renege of flexibility levels - characterised by amount of flexible generation, demand side response, energy storage and interconnectivity.
• No viable renewables in low flexibility case, viable renewables supplying approx 2/3 of electricity in fully flexible case.
• Savings chiefly arise through reduced capital investment in nuclear and CCS - which more than compensates for increased investment in renewable energy sources.

Question 28

What has been the impact of off-grid solar + lead acid batteries vs diesel in Africa?

Answer 28

Solar PV + lead acid battery storage has been cost-competitive with grid connection and off-grid diesel generation in much of Africa since 2011.

Question 29

What has been the impact of off-gird solar PV/Li-ion/diesel hybrid system in India?

Answer 29

• Lowest cost system generates 100% of electricity from diesel according to 2015 cost estimates, at approx 1,000 gCO2/kWh
• Lowest cost system projects to supply approx 80% from solar from 2020 at 500 gCO2/kWh
• Meeting very high proportions of demand (>95%) with solar PV + storage may have a higher energy intensity than a hybrid diesel system owing to the carbon footprint of a large solar PV panel.

Question 30

What are the main policy/regulation challenges for energy storage?

Answer 30

• Policy structures to access multiple streams of income - many technologies are technically able to offer a range of services (e.g EFR and capacity) but cannot bid for them simultaneously.
• Policy structures to access multiple streams of income - most domestic consumers pay a flat-rate for electricity which does not vary with what that unit of electricity cost to generate. Roll-out of smart meters could help.
• Removal of regulatory barriers, e.g. double charging of fees as generator and consumer etc
• Technology support - R&D, demonstration projects, deployment
• Contract length - contracts lasting just a few years allow providers the option to replace technology at the end of the contract period, but some technologies have proven or projected lifetimes of 10’s of years. Short contract lengths also likely to promote cost-saving developments over increasing the lifetime of the technologies.
• Regulation around environmental impact - whilst some regulations exist around battery disposal, further policies and regulations could help to reduce environmental impact of electricity storage technologies.